1. Field of the Invention
The present invention relates to an electron beam irradiating apparatus for use in semiconductor device fabrication, and more particularly, to an electron beam irradiating apparatus having a cathode plate formed of a non-metal conductive material.
2. Description of the Related Art
Semiconductor devices are comprised of conductive layers used to form interconnections and insulation layers for electrically insulating neighboring interconnections. With increases in the operating speed of highly integrated semiconductor devices, the need for a multi-layered interconnection techniques has increased. This has increased the interest in planarization technologies for inter-layer dielectric (ILD) films and inter-metal dielectric (IMD) films. At present, a technique for reflowing an oxide layer, which contains impurities such as boron and/or phosphorous, for example, a borophosphosilicate glass (BPSG) layer, at a high temperature of 850xc2x0 C. or more, has been adopted to planarize ILD films. Also, for IMD film planarization, a spin-on-glass (SOG) technique has been used. Under this technique, a liquid spin-on-glass is spun on a semiconductor substrate on which a lower metal interconnection has been formed. Then the glass is cured at a low temperature of 500xc2x0 C. or less. In the curing of the SOG layer, a UV irradiation or electron beam irradiation technique has been widely used.
In the electron beam irradiation approach, the energy of the electron beams can readily be controlled, compared to UV irradiation. Also, the electron beam irradiation can be applied in baking a photoresist layer as well as in the curing of the SOG layer. Systems used in electron beam irradiation are very similar to sputtering systems.
A conventional electron beam irradiating apparatus includees a cathode plate on the top of an airtight chamber and a susceptor, on which a wafer will be mounted, disposed on the bottom of the chamber, facing the cathode plate. The cathode plate is formed of a conductive metal material, for example, aluminum (Al) or an aluminum alloy. Also, a grid plate, i.e., an anode plate, through which electron beams emitted from the cathode plate can pass, is disposed between the cathode plate and the susceptor. Also, a gas injecting ring for supplying a process gas such as an inert gas into the chamber is provided.
In the operation of the conventional electron beam apparatus, the pressure of the chamber is maintained at a low pressure which is lower than atmospheric pressure, and a process gas is allowed to flow into the chamber through the gas injecting ring. A high negative voltage, for example, a voltage of xe2x88x92500 to xe2x88x923,000 volts, is applied to the cathode plate, while a low negative voltage, for example, a voltage of 0 to xe2x88x92100 volts, is applied to the grid plate. At this time, the process gas is slightly ionized due to the low pressure in the chamber and gamma rays which spontaneously generate in the chamber, and the positive process gas ions move to the grid plate due to the low negative voltage applied to the grid plate. The positive ions pass through holes of the grid plate and are accelerated toward the cathode plate to which the high negative voltage is applied. As the accelerated positive ions hit the surface of the cathode plate, secondary electrons are emitted from the cathode plate. At this time, metal atoms such as aluminum (Al) atoms which form the cathode plate, as well as the electrons, may be emitted. The electrons emitted from the cathode plate reach the ionized process gas through the grid plate. Some electrons further ionize the process gas into positive and negative ions, and the remaining electrons reach the surface of the wafer loaded onto the susceptor. Also, the metal atoms emitted from the cathode plate, i.e., Al atoms, reach the wafer surface.
As described above, the conventional electron beam irradiating apparatus generates metal atoms as well as electrons, so that the wafer surface is subjected to contamination by the metal atoms. In particular, when a photoresist layer is baked or a SOG layer is cured using the conventional electron beam irradiating apparatus, prior to metal interconnection formation, the electrical properties of MOS transistors can vary. For example, the characteristics of the junction leakage current of sources/drains are deteriorated or the threshold voltage characteristics can change. In the case where the wafer surface is contaminated by metals the electrical properties and the reliability of MOS transistors are sharply deteriorated. Also, in the case where the wafers contaminated by the metal atoms are immersed in a bath containing a rinsing solution, the rinsing bath may be also contaminated by the metal atoms. Thus, succeeding wafers which are immersed in the contaminated bath are exposed to contamination left behind by preceding wafers, which cumulatively increases.
It is an object of the present invention to provide an electron beam irradiating apparatus capable of preventing the surface of wafers from becoming contaminated by metal atoms.
To achieve the object, the present invention provides an electron beam irradiating apparatus which includes a chamber having an opening at its top side. A cathode plate is disposed to cover the opening of the chamber and has a bottom surface which faces the bottom of the chamber and is formed of a non-metal conductive material. A susceptor is disposed on the inner bottom surface of the chamber. A grid plate is disposed between the cathode plate and the susceptor.
In one embodiment, a gas injection ring is included to allow a process gas to enter the chamber. In one particular embodiment, the gas injection ring is disposed between the grid plate and the susceptor.
In one embodiment, the cathode plate and the chamber are electrically insulated from each other. Also, the grid plate can be electrically insulated from the chamber and the cathode plate.
In one embodiment, the cathode plate is a single cathode plate formed of the non-metal conductive material alone. The non-metal conductive material can be silicon. In another embodiment, the cathode plate is a double cathode plate in which at least a lower surface of the cathode plate, facing the bottom surface of the chamber, is made of the non-metal conductive material. Specifically, the double cathode plate can have an upper cathode plate made of metal such as aluminum (Al) or an Al alloy and a lower cathode plate made of the non-metal conductive material such as silicon.
In one embodiment, the chamber is connected to ground terminal, so that the susceptor and a wafer loaded on the susceptor are grounded. The chamber can be connected to a vacuum pump. The vacuum pump functions to control the inner pressure of the chamber at a lower pressure than the atmospheric pressure.
In accordance with the invention, a wafer is loaded onto the susceptor, and the inner pressure of the chamber is maintained at a lower pressure than the atmospheric pressure by operation of the vacuum pump. A process gas is flowed into the chamber through the gas injection ring. A high negative voltage of xe2x88x92500 to xe2x88x9230,000 volts is applied to the cathode plate, while a low negative voltage of 0 to xe2x88x92500 volts is applied to the grid plate. At this time, the process gas injected into the chamber is slightly ionized by gamma rays, which spontaneously generate in the chamber, thus producing positive and negative ions. The produced positive ions, due to the electric field created by the voltages applied to the grid and cathode plates, pass through the grid plate and are accelerated toward the cathode plate. Finally the accelerated positive ions hit the cathode plate, thus inducing emission of secondary electrons from the surface of the cathode plate. Since the bottom surface of the cathode plate, which is struck by the positive ions, is formed of a non-metal conductive material, for example, silicon, emission of metal atoms does not occur with the emission of the secondary electrons from the cathode plate surface. The electrons emitted from the cathode plate reach the wafer surface with a predetermined energy, according to the electric field produced by the voltages applied to the grid and cathode plates. Accordingly, a predetermined material layer formed on the wafer surface, for example, a spin-on-glass (SOG) layer, is cured, resulting in a material layer which has similar properties to those of silicon oxide layers.
The electron beam irradiating apparatus according to the present invention uses a cathode plate whose bottom surface is made of a non-metal material, so that emission of the metal atoms from the cathode plate can be eliminated. Thus, the contamination of the wafer surface by metal atoms during electron beam irradiation can be avoided.